Radiography systems and methods are widely used, particularly for medical imaging and diagnosis. Radiography systems generally create two-dimensional projection images through a subject's body. A radiation source, such as an X-ray tube, irradiates the body from one side. A collimator, generally adjacent to the X-ray source, limits the angular extent of the X-ray beam, so that radiation impinging on the body is substantially confined to a cone-beam/fan-beam region (i.e., an X-ray projection volume) defining an image volume of the body. At least one detector on the opposite side of the body receives radiation transmitted through the body substantially in the projection volume. The attenuation of the radiation that has passed through the body is measured by processing electrical signals received from the detector.
X-ray projection images having high spatial resolution are desirable in order to visualize fine details in the image. However, spatial resolution can be limited by the detector pixel size. Additionally, the spatial resolution can be limited by the spatial extent of the X-ray source (i.e., the focal spot size), and the geometrical arrangement among the source, the imaged object, and the X-ray detector. The short wavelength of X-rays minimizes the effects of diffraction. However, as the size and spacing of the pixels of the X-ray detector array continue to get smaller with improvements in detector technology, improvements to decreasing the size of the focal-spot size in X-ray sources have failed to keep pace, resulting in the X-ray source being the limiting factor in resolution. Due to intrinsic material and thermal constraints in X-ray tubes, the focal spot size has remained relatively constant, whereas the critical dimensions of the X-ray detector array (e.g. the width and spacing of the detector elements in the array) have decreased over time, until now the spatial resolution for X-ray detects is smaller than the width of the point-spread function of X-ray sources operating under typical clinical settings.
The focal spot is the point where the electron beam strikes a target within an X-ray tube. Thus, the focal-spot size is determined by the size of the electron beam and the aspect angle between the surface struck by the X-rays and the direction from the X-ray source to the target. A small focal-spot size improves the resolution of the X-ray imaging, resulting in more detailed images. However, it is often difficult to use a small focal-spot size due to the constraints imposed by X-ray tube loading necessary to achieve a desired exposure and signal-to-noise-ratio (SNR).
The width of the focal spot is not the only factor determining the point-spread function. Additionally, the point-spread function is affected by the ratio between object-imager distance (OID) and source-imager distance (SID). The closer an object is to the detector and the farther away the object is from the source, the smaller the point-spread function becomes, resulting in less blurring in the generated image. Thus, the spatial resolution can be improved by making the ratio SID:OID large. This can be accomplished by keeping the OID to a minimum, e.g., by keeping the object close to the detector. Further, the ratio SID:OID is large when the SID is large by positioning the object a long distance from the X-ray source. However, practical constraints impose bounds on how large the ratio SID:OID can be for clinical applications.
In clinical X-ray imaging systems, the focal-spot size is typically on the order of one millimeter, which is large enough to be the limiting factor for the X-ray image resolution. High resolution detectors with a pixel size significantly less than one millimeter create potential for higher resolution X-ray imaging, but this potential cannot be fully realized without overcoming the practical limitations imposed by the size of focal spots and magnification factors. Tube design limitations present obstacles to improve X-ray imaging resolution without degrading the SNR by decreasing the exposure. A method of increasing resolution without significantly degrading SNR in X-ray images would be advantageous.
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as conventional art at the time of filing, are neither expressly nor implicitly admitted as conventional art against the present disclosure.